In utility boilers and other devices in which fossil fuels are burned, safety considerations dictate that flame be continuously present near each point at which fuel is introduced. Toward this end various devices have been proposed and used which monitor the presence or absence of flame and close down the fuel supply when flame disappears. In the design of these flame detectors, particularly those for use in multiple-burner furnaces, a problem which must be dealt which is how to discriminate between the presence of flame at the location to be monitored and the presence of flame at other locations. It is to this problem that the method and apparatus of the present invention is addressed.
It is a commonplace within the art to detect electromagnetic radiation, particularly that in the ultraviolet range, and use the intensity level detected as an indication of the presence or absence of flame. This type of device, of course, responds not only to radiation reaching it from the flame to be monitored, but also responds to any other electromagnetic radiation reaching it, including any radiation which reaches it from nearby burners. Therefore, it is necessary to design the device in such a manner that it will not respond to an intensity of radiation that is below a level that can only be achieved by the flame to be monitored. This method will always work only if a range of intensities exists which cannot be achieved by the background flame and only if the flame to be monitored, when present, will not decrease below that range. However, if the range of intensities of radiation from the flame to be monitored overlaps the range of intensities of radiation from background flame, then conditions exist which will cause this method to give incorrect readings. In addition, smoke and the accumulation of dirt on the face of the detector may render the device sufficiently insensitive that it will shut off fuel even when a fairly intense flame is present. Accordingly, it may be seen that effective use of a device of this type is a sometime thing.